1. Field of the Invention
The present invention relates to a process for the preparation of p-amino-phenols by catalytic hydrogenation of the aromatic nitro compounds on which they are based in a reaction medium of aqueous sulphuric acid in the sense of a Bamberger rearrangement, the reaction additionally being carried out in the presence of a water-miscible organic solvent (cosolvent).
p-Amino-phenols are of importance as intermediate products for the preparation of dyestuffs, pharmaceutical active compounds and plant protection agents.
2. Description of the Related Art
The preparation of p-amino-phenols by hydrogenation of nitroaromatics having a free p-position in an aqueous acid medium using hydrogenation catalysts is known as the Bamberger reaction in various variants (Houben-Weyl, 4th Edition, Volume VI/lc (1976), pages 85 to 117). Aqueous sulphuric acid is in general chosen as the reaction medium for the process. The rate of reaction in this process substantially depends on the solubility of the organic nitro compounds employed as the starting materials in the reaction medium or their wettability by the aqueous system. However, since this solubility or wettability of the nitroaromatics usually used as starting materials is exceptionally low, this matter represents a critical point which makes the usability of the Bamberger reaction more difficult and limits and in some cases completely prevents it.
For the reasons mentioned, in the past there has been no lack of attempts to improve the wetting of the organic nitro compound and in this way to exert a favorable influence on the course of Bamberger reactions. Thus, for example, U.S. Pat. No. 3,383,416 describes the addition of dodecylethylammoniumchloride and U.S. Pat. No. 4,307,249 describes the addition of dimethyl-alkylamine oxides as surface-active substances. According to U.S. Pat. No. 3,535,382, the addition of polyethers based on phenol of the type EQU R--C.sub.6 H.sub.4 --O--(--CH.sub.2 --CH(OH)--CH.sub.2 --O--).sub.x --CH.sub.2 --CH(OH)--CH.sub.2 OH
is said to be of advantage. DE-AS (German Published Specification) 1,904,574 describes a process in which the nitrobenzene used as the starting material is pumped into the reaction mixture in the course of the reaction in order to avoid a decrease in the catalytic activity of the catalyst by (as yet) unreacted, virtually water-insoluble nitrobenzene.
Disadvantages of the process variants according to the US patents mentioned are the problems associated with the auxiliaries, since these auxiliaries make isolation and purification of the desired reaction product after the reaction has ended more difficult, in general cannot be reused, magnify the waste problem and therefore impair the profitability. In the process described in the DE-AS (German Published Specification) mentioned, expensive control of the metering device is additionally necessary. Another serious disadvantage of all the abovementioned processes is that, under the conditions described, only the reaction of nitrobenzene itself and a few other nitroaromatics which still have a relatively good water-solubility and in general contain not more than one other substituent leads to satisfactory selectivities and space-time yields, but that more highly substituted nitroaromatics can be reacted only poorly and incompletely.
JP 51/110,528 (1976) describes Bamberger reactions with the addition of water-immiscible solvents, such as toluene, dibutyl ether, 1,1,1-trichloro-ethane or octanol. However, these do not improve in principle the problem of the poor wettability of the organic nitro compound by the aqueous-acid phase.
Another difficulty which often occurs in reactions of the Bamberger type is the fact that in addition to the optionally substituted p-amino-phenol required, significant amounts of the optionally substituted aminoaromatic are formed. The occurrence of this byproduct, which can assume considerable proportions which reduce profitable utilisation of the process in certain cases, is in general to be attributed to a reduction which competes with the Bamberger reaction, such as usually proceeds during the action of hydrogen on nitro compounds in the presence of noble metal catalysts. In order to influence the ratio of the desired aminophenol to the undesirable aminoaromatics advantageously, certain substances, such as, for example, dimethyl sulphoxide, are added to the reaction mixture, these partly poisoning the catalyst and moderating or reducing its catalytic activity and thus leading to a reduced formation of aminoaromatics.
A problem with this procedure is the fact that an additive is introduced into the reaction, the removal of which in the course of working up often presents problems. Furthermore, catalysts deactivated in this way cannot be re-used, which is an additional disadvantage. An example of this procedure is JP 01/121,254; cited in C. A. 111, 173 752.
The object was therefore to discover a process for the preparation of the optionally substituted 4-amino-phenols mentioned below, which on the one hand allows good wetting of the starting material in the course of the Bamberger reaction and thus leads to reaction times and yields which are of economic interest, avoids or at least largely suppresses the formation of undesirable byproducts and is carried out without the use of catalyst poisons, so that working up problems are avoided and it is ensured that the catalyst is reusable.
This object is achieved by the process according to the invention.
In contrast to the procedures described above, the process according to the invention is characterized in that it improves the use of the Bamberger reaction for the preparation of the optionally substituted 4-aminophenols mentioned below from the corresponding nitroaromatics and dispenses with the addition of catalyst poisons, so that the catalyst separated off when the reaction has ended can be reused again in the process according to the invention.
The process according to the invention is characterized in that a water-miscible organic solvent (cosolvent) is added to the reaction mixture. Such cosolvents improve the mixing of the organic and inorganic phase quite considerably, in this way have an accelerating effect on the course of the reaction and can easily be recovered when the reaction has ended, and if appropriate reused.
Finally, another surprising advantage of the process according to the invention is that sometimes considerable selectivity improvements in favor of the desired substituted p-amino-phenols are observed when such cosolvents are added.